The invention relates to a measurement setup for measuring physical values which has two or more outputs with different resolution levels comprising a piezoelectrical sensor and two or more amplifiers.
Measurement setups essentially comprise a sensor for sensing and converting a signal and an amplifier for amplifying the signal before it is transmitted to the output. Depending on the application, different amplifications or conversion factors of measurement signals are required. Usually an amplifier can be adjusted according to need: a high sensitivity can be chosen whereby the resolution will be very high but the total measurement range very narrow, or a low sensitivity can be selected so that the resolution is very low but the total measurement range is very broad.
Certain applications, however, require different resolution rates at different times. For example, in monitoring the distribution of forces of injection moulding or die casting machines while the mould halves are clamped for the injection and cooling operations very high maximum forces are measured. It must be assured, on the other hand, that during the closing operation the space between the mould halves is empty, i.e. that in particular no part to be removed has been caught and could damage the mould during the closing. For this purpose, the resolution of the force signal is required to be about 50 to 100 times higher than that of the signal in the closed state.
Another application example of the requirement to measure both a highly resolving weak signal and to monitor and evaluate a very strong signal is the field of ballistics.
Therefore, there is a need to monitor both high and low amplitudes with nearly identical resolutions relative to the amplitudes.
According to the prior art, for example, two measuring chains each having a sensor and an amplifier are used for this purpose which can be set individually. While one of the measuring chains is suitable for reproducing low forces in a low range and with a high resolution the other measuring chain is adapted to reproducing high forces with a high amplitude and low resolution. The first measuring chain turns to saturation with increasing forces. A disadvantage of a setup of this type is the space requirement for the two sensors, the need of double installation and the costs of two complete measuring chains.
FIG. 1 represents two measuring chains according to the prior art which are independent of each other and arranged in parallel each measuring chain consisting of a sensor 10 and 11 and an amplifier 20 and 21 wherein the amplifiers have different sensitivities so that the charge signals Q1 and Q2 sensed at the sensors 10 and 11, for example due to forces acting thereon, have different signal amounts and sensitivities at the outputs A1 and A2, respectively, of the amplifiers 20 and 21. Two complete setups are required for these measuring chains, in particular two sensors 10 and 11 both of which must be installed. Although this solution is very costly it has been employed.
Another possibility is to use only one measuring chain wherein the amplifier must be switched over to a new desired sensitivity at an appropriate time. A disadvantage of this setup is the discontinuity of the course of the measurement as well as the difficulty to find the right point of time for switching over.
FIG. 2 shows another setup of a measuring chain according to the prior art. This measuring chain comprises only one sensor 10 and an amplifier 20, 21 with a setting means 30 for the amplifier 20, 21 whereby depending on the setting the only one terminal A can correspond to the sensitivity of the first sensor 20 or of the second sensor 21 of FIG. 1. This arrangement has the disadvantage that at any give time a charge signal Q can be amplified only with a high sensitivity or with a low sensitivity and that indication means are required which determine time of switching over.
It is also possible to evaluate a partial signal superposed on an electrical signal as already described in CH 681755. However, since the partial signal is not related to the same zero point as the basic signal this procedure is unsuitable for the applications and requirements described herein.